Pembrolizumab
[CAS 1374853-91-4]

Identification

• Classification: API >> Antineoplastic agents
• Name: Pembrolizumab
• Synonyms: Keytruda; Lambrolizumab; Lanbrolizumab; MK 3475; Merck 3475
• Molecular Formula: C₆₅₀₄H₁₀₀₀₄N₁₇₁₆O₂₀₃₆S₄₆
• Molecular Weight: 146286.29
• Protein Sequence: Heavy Chain Sequence$$nl$$QVQLVQSGVEVKKPGASVKVSCKASGYTFTNYYMYWVRQAPGQGLEWMGGINPSNGGTNF$$nl$$NEKFKNRVTLTTDSSTTTAYMELKSLQFDDTAVYYCARRDYRFDMGFDYWGQGTTVTVSS$$nl$$ASTKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSS$$nl$$GLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNTKVDKRVESKYGPPCPPCPAPEFLGGPSV$$nl$$FLFPPKPKDTLMISRTPEVTCVVVDVSQEDPEVQFNWYVDGVEVHNAKTKPREEQFNSTY$$nl$$RVVSVLTVLHQDWLNGKEYKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEEMTK$$nl$$NQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEG$$nl$$NVFSCSVMHEALHNHYTQKSLSLSLGK$$nl$$$$nl$$Light Chain Sequence$$nl$$EIVLTQSPATLSLSPGERATLSCRASKGVSTSGYSYLHWYQQKPGQAPRLLIYLASYLES$$nl$$GVPARFSGSGSGTDFTLTISSLEPEDFAVYYCQHSRDLPLTFGGGTKVEIKRTVAAPSVF$$nl$$IFPPSDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLS$$nl$$STLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNRGEC
• CAS Registry Number: 1374853-91-4
• EC Number: 807-012-2

Properties

• Solubility: 22.5-27.5 mg/ml (histidine buffer)

Safety Data

• Hazard Symbols: GHS08 Danger
• Risk Statements: H360-H372
• Safety Statements: P203-P260-P264-P270-P280-P318-P319-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Reproductive toxicity	Repr.	1B	H360
Specific target organ toxicity - repeated exposure	STOT RE	1	H372

chemBlink Chemical Story

Pembrolizumab, CAS 1374853-91-4, is a humanized monoclonal antibody that changed an old assumption about cancer treatment. Many anticancer drugs are designed to attack the tumor directly: damage its DNA, interrupt cell division, block an essential enzyme, or inhibit a growth signal. Pembrolizumab takes a different approach. Its principal target is not a molecule that makes up the tumor. It binds to PD-1, a regulatory receptor on immune cells, and interferes with a molecular "brake" that tumors can exploit to protect themselves from immune attack.

The immune system faces a difficult engineering problem. T cells must be powerful enough to recognize and destroy infected or abnormal cells, yet controlled enough to avoid attacking healthy tissues indiscriminately. Activation signals are therefore balanced by inhibitory signals known as immune checkpoints.

One of these checkpoints involves programmed cell death protein 1, or PD-1. Activated T cells can express PD-1 on their surfaces. When PD-1 binds to its ligands PD-L1 or PD-L2, intracellular signaling reduces aspects of T-cell activity. This mechanism is useful in normal physiology because it helps restrain excessive immune responses and limit tissue damage.

Cancer can turn this protective system to its own advantage.

Many tumors or cells within the tumor environment can express PD-L1. When PD-L1 engages PD-1 on a T cell, the resulting inhibitory signal can reduce the T cell's ability to attack the tumor. The cancer has not necessarily become invisible. Instead, it can take advantage of a normal regulatory pathway that tells the immune response to slow down.

Pembrolizumab was designed to interfere with that conversation.

It is a humanized IgG4 kappa monoclonal antibody that binds PD-1 and blocks its interaction with PD-L1 and PD-L2. By preventing those ligands from activating the PD-1 inhibitory pathway, pembrolizumab can release some of the restraint on antitumor T-cell responses.

This distinction is fundamental. Pembrolizumab does not simply "stimulate the immune system" in a general sense, nor does it directly poison a cancer cell in the way a conventional cytotoxic drug might. It blocks a specific regulatory interaction. The immune system must still recognize and attack the tumor.

The idea represented a major change in cancer pharmacology. For much of the twentieth century, anticancer therapy focused heavily on killing rapidly dividing cells. Later generations of targeted drugs attacked particular molecular abnormalities driving tumor growth. Immune-checkpoint therapy added another strategy: instead of concentrating exclusively on weaknesses inside the cancer cell, manipulate the regulatory relationship between cancer and the immune system.

Clinical development moved rapidly. Pembrolizumab, initially known as MK-3475, produced striking and sometimes durable responses in patients with advanced melanoma during early clinical studies. In September 2014, the U.S. Food and Drug Administration approved pembrolizumab for certain patients with unresectable or metastatic melanoma whose disease had progressed after previous therapy. It became the first PD-1 inhibitor approved in the United States.

Melanoma was only the beginning.

Subsequent clinical trials established roles for pembrolizumab in numerous malignancies, including forms of lung cancer, head and neck cancer, classical Hodgkin lymphoma, urothelial cancer, colorectal cancer, gastric and gastroesophageal junction cancer, cervical cancer, endometrial cancer, breast cancer, renal cell carcinoma, and others. Depending on the disease and clinical setting, pembrolizumab may be used alone or in combination with chemotherapy, targeted agents, or other treatments.

An especially important development was that pembrolizumab helped challenge another traditional rule of oncology: drugs were usually approved for cancers defined primarily by the organ where the tumor originated.

A lung cancer was treated as lung cancer; a colon cancer as colon cancer.

But tumors arising in different organs can sometimes share molecular features that strongly influence their interaction with the immune system. Defects in DNA mismatch repair, for example, can produce microsatellite instability and allow large numbers of mutations to accumulate. Those mutations can generate abnormal proteins and potentially create additional targets recognizable by immune cells.

This led to treatment strategies based on biomarkers such as mismatch-repair deficiency and high microsatellite instability rather than solely on anatomical origin. Pembrolizumab subsequently received approvals in biomarker-defined settings, and tumor mutational burden also became relevant for particular indications.

The conceptual change was profound. The question could sometimes shift from "Where did this cancer start?" toward "What biological properties does this cancer have?"

Checkpoint inhibition, however, has an unavoidable consequence. PD-1 exists for a reason.

If an inhibitory pathway that normally helps restrain immune responses is blocked, activated immune cells can sometimes attack healthy tissues as well as tumors. Pembrolizumab can therefore cause immune-mediated adverse reactions involving organs including the lungs, colon, liver, endocrine glands, kidneys, and skin. Some reactions can be severe or life-threatening.

This apparent paradox is central to understanding the drug. Its therapeutic effect and some of its characteristic toxicities arise from the same fundamental action: removing an immune restraint.

For many conventional drugs, an adverse effect may seem unrelated to the intended target. With checkpoint inhibitors, the relationship can be unusually direct. Release the immune brake enough and antitumor activity may improve; release immune restraint in the wrong tissue and inflammation or autoimmunity-like injury can result.

Pembrolizumab is also chemically very different from an ordinary small-molecule drug. A molecule such as aspirin can be represented by a compact structural formula and manufactured through a sequence of organic chemical reactions. Pembrolizumab is a large protein assembled from amino-acid chains, folded into a precise three-dimensional antibody structure, connected by disulfide bonds, and produced using living cells.

Its identity therefore cannot be adequately understood from a simple two-dimensional structural drawing. Sequence, higher-order structure, biological activity, purity, aggregation state, and manufacturing conditions all matter. Pembrolizumab belongs to the world of biologic medicines, where the product is not merely synthesized but expressed, folded, purified, and characterized.

The antibody's familiar Y-shaped architecture also gives it extraordinary molecular recognition. At the tips of the antibody are variable regions engineered to recognize PD-1 with high specificity. Binding at this microscopic interface can ultimately alter interactions among immune cells and tumor cells throughout the body.

That enormous amplification of scale is one of the most remarkable aspects of antibody therapy. A molecular recognition event measured in nanometers can change the behavior of immune-cell populations, shrink a tumor visible on a medical scan, and in some patients produce responses that persist long after treatment began.

Pembrolizumab therefore represents more than another anticancer drug. It illustrates a change in what the word "target" can mean in pharmacology.

The target does not always have to be the cancer cell itself.

Sometimes the more effective strategy is to find the molecular signal the cancer is using to tell the immune system, "Do not attack."

Then block the signal and let the immune system continue the conversation.

References

1. U.S. Food and Drug Administration. Prescribing and regulatory information for pembrolizumab. Mechanism of action, clinical indications, and immune-mediated adverse reactions.

2. U.S. Food and Drug Administration. Approval history for pembrolizumab, including the original 2014 approval for unresectable or metastatic melanoma.

3. Hamid, O. et al. (2013). "Safety and Tumor Responses with Lambrolizumab (Anti-PD-1) in Melanoma." New England Journal of Medicine, 369, 134-144.

4. Robert, C. et al. (2015). "Pembrolizumab versus Ipilimumab in Advanced Melanoma." New England Journal of Medicine, 372, 2521-2532.

5. Published clinical and immunological literature on the PD-1/PD-L1 immune-checkpoint pathway and pembrolizumab therapy.

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